1. Field of the Invention
The present invention relates to a surface acoustic wave device, and in particular to a surface acoustic wave device in which any one of an input and output has balanced or differential terminal pair.
2. Description of the Related Arts
The surface acoustic wave device is widely used as a filter in a high frequency circuit of a radio apparatus represented by a portable telephone or the like. In recent years, in the high frequency circuit of this radio apparatus, an integrated circuit element (IC) having a balanced or differential input and output has been used.
On the contrary, a filter using a conventional surface acoustic wave device (hereinafter appropriately referred to as a surface acoustic wave filter) is unbalanced in an I/O terminal. For this reason, for example, as shown in FIG. 1, in the case of connecting with a mixer circuit IC 3, an unbalanced-balanced transforming part which is called a balun, or a transformation circuit 2 constituted by separate parts is necessary between a surface acoustic wave filter 1 and the mixer circuit IC 3.
Furthermore, the surface acoustic wave filter normally has an I/O impedance of 50 xcexa9, and on the other hand, in many cases, an impedance of the mixer circuit IC 3, etc. having a balanced terminal pair is high up to about 100 to 200 xcexa9, and an impedance transformation circuit was required for connecting such the IC with the surface acoustic wave filter.
However, this leads to an increase in the number of using circuit parts in the radio apparatus. Furthermore, for a demand for more downsizing, a design for space-saving is demanded.
It is therefore an object of the present invention to realize a small-sized surface acoustic wave device having an unbalanced-balanced transformation function and a function of impedance transformation.
In order to achieve the above object, according to a first aspect of the present invention there is provided a surface acoustic wave device, comprising an input interdigital transducer and an output interdigital transducer, disposed on a surface acoustic wave propagation path of a piezoelectric substrate, wherein when an aperture length of an electrode finger of the input or output interdigital transducer is denoted by X, the output or input interdigital transducer has two divided interdigital transducers having the electrode finger in which each aperture length is denoted by substantially X/2, and wherein the two divided interdigital transducers are serial-connected, and the electrodes of the respective electrode fingers are led from the two divided interdigital transducers, and are disposed so that two output and input signals connected to a balance terminal pair have a different phase at 180xc2x0.
In order to achieve the above object, according to a second aspect of the present invention there is provided a surface acoustic wave device, comprising a plurality of interdigital transducers disposed on a surface acoustic wave propagation path of a piezoelectric substrate, and a reflecting electrode disposed at both the sides, wherein the plurality of interdigital transducers contain a first type of interdigital transducer and a second type of interdigital transducer disposed alternately, wherein when an aperture length of an electrode finger of the first type of interdigital transducer is denoted by X, each of the second type of interdigital transducers has two divided interdigital transducers having an electrode finger in which each aperture length is denoted by substantially X/2, and wherein the first type of interdigital transducer is connected to an unbalanced input or output terminal pair, and the two divided interdigital transducers are serial-connected, and the electrodes of the respective electrode fingers are led from the two divided interdigital transducers, and are connected to a balanced terminal pair, and the respective electrode fingers of the two divided interdigital transducers are disposed so that phases of signals in the balanced terminal pair are different at 180xc2x0.
Preferably, in the first or second aspect of the present invention, a position of the electrode finger at a side of connecting with the balanced terminal is mutually slid in half-waves, in the two divided interdigital transducers.
Preferably, in the first or second aspect of the present invention, the connection part of the two divided interdigital transducers is connected to ground.
Preferably, in the second aspect of the present invention, the plurality of interdigital transducers constitute a double mode filter by three interdigital transducers.
Preferably, in the second aspect of the present invention, the plurality of interdigital transducers are five or more interdigital transducers, constituting a multi-electrode filter.
a surface acoustic wave device has two or more filters are cascade-connected, of which the outermost filter is constituted by the surface acoustic wave device according to the above first or second aspect of the present invention, and a balanced terminal pair for an input or output.
Preferably, in the aspect that the two or more filters are cascade-connected, a cascade-connection is conducted between the two or more mutual filters by the plurality of connection parts of the interdigital transducer constituting each filter, and a phase of the filter is reversed in each neighboring connection part of the plurality of connection parts.
Preferably, the piezoelectric substrate may be a 40xc2x0 to 44xc2x0 rotated Y-X LiTaO3 in any of the above aspects of the present invention.
More, preferably, the piezoelectric substrate may be a 66xc2x0 to 74xc2x0 rotated Y-X LiNbO3 in any of the above aspects of the present invention.